Balanced devices operating at high frequencies including, but not limited to, microwave frequencies, are becoming more and more prevalent in modern systems, especially communication systems. Concomitant with the use of such devices is a need to measure a performance of balanced devices. As with single-ended (i.e., non-balanced) microwave devices, balanced microwave devices may be characterized using S-parameters. However, in the case of balanced devices, so-called ‘mixed mode’ S-parameters are generally used to characterize the device instead of the conventional or single-ended S-parameters. Mixed mode S-parameters relate differential and common mode signals, which are applied to ports of the balanced device, to differential and common mode responses at the ports.
Conceptually, a multiport vector network analyzer (VNA) that provides a single-ended (e.g., non-differential) stimulus signal to a device under test (DUT) may be used to measure mixed-mode S-parameters of a balanced DUT. Specifically, since a balanced device may be viewed as a generalized multiport device, the multiport VNA may be used to measure mixed-mode S-parameters of the balanced DUT by applying a single-ended signal separately to each port of the balanced DUT and measuring a set of single-ended S-parameters for each port. After applying and measuring is performed for each port, a complete set of single-ended S-parameters is produced. The complete set of measured single-ended S-parameters then can be converted to mixed-mode S-parameters using modal decomposition to completely characterize a balanced device. See for example, Bockelman et al., U.S. Pat. Nos. 5,495,173 and 5,751,153, incorporated herein by reference.
Unfortunately, some balanced devices, especially active balanced devices and those exhibiting certain non-linearities, behave differently in response to a single-ended stimulus signal than to a true differential stimulus signal. Therefore, the single ended S-parameters measured by the conventional VNA may not accurately reflect a performance of the balanced device in the presence of a differential stimulus signal. In a worst case, the balanced device may even fail to operate (e.g., become unstable resulting in unwanted oscillations) or be damaged when presented with a single-ended stimulus signal.
To address the measurement of such balanced devices, various approaches have been developed that seek to directly measure the mixed-mode S-parameters of a balanced DUT using true or simulated differential stimulus signals. Among the various approaches are methods employing a broadband balun (i.e., either physical or simulated balun) and methods using a hybrid junction. Each of these methods employs a device (e.g., a balun or a hybrid junction) between the VNA and the DUT to essentially transform the single-ended stimulus signal into a differential signal. In another approach, a pure mode network analyzer (see Bockelman et al., cited supra) employs as a stimulus signal source a composite signal generator that sequentially switches between a common-mode (i.e., in-phase) stimulus signal and a differential-mode (i.e., out of phase) stimulus signal.